This application is based upon and claims the benefit of priority of Japanese Patent Applications No. H.10-363027 filed on Dec. 21,1998 and No. H-324569 filed on Nov. 15, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a spark plug for internal combustion engine provided with a straight pillar type ground electrode extending perpendicularly to a metal housing, in particular, applicable to the engine for a co-generation system, a delivery pump for pressurized gas and a vehicle.
2. Description of Related Art
A conventional spark plug, as shown in FIG. 16, has generally a center electrode J4 fitted through an insulator J3 into a housing J1 and a ground electrode J2 fixed to the housing J1. The center electrode J4 partly exposed out of the end of the insulator J3 faces the ground electrode J2. The ground electrode J2 is curved in a letter L shape to form a spark discharge gap between a leading end of the center electrode J4 and a leading end of the ground electrode J2.
In the engine for the co-generation system, for example, a higher heat resistance characteristic of the spark plug is required so that a thickness of the ground electrode J2 may be relatively thick and material of the ground electrode J2 is relatively hard. Therefore, a length h of the ground electrode J2 protruded into a combustion chamber of the engine tends to be longer because of difficulty of sharply bending into the letter L shape. As the protruded length h into the combustion chamber is longer, temperature of the ground electrode becomes higher and the heat resistance of the spark plug is more rapidly deteriorated.
Further, in the spark plug, for example, applicable to the engine for the co-generation system or the pressurized gas delivery pump, a periodical spark discharge gap adjustment is required because an electrode consumption by the spark discharge is more rapidly made.
The present invention has been made in view of the above mentioned problems or requirements, and an object of the present invention is to provide a spark plug having a straight pillar type ground electrode whose protruding length into the combustion chamber is relatively short so that the temperature of the ground electrode may be kept low and a longer life time of heat resistance may be assured. In addition, the spark plug is provided with an easy gap adjustment means.
To achieve the above object, the spark plug is composed of a center electrode, a housing holding and being insulated with the center electrode so as to expose a leading end of the center electrode out of an end thereof, a ground electrode bonded at a first leading end thereof to the end of the housing and extending straight from the housing substantially perpendicular to an axis of the housing so as to form a spark discharge gap between the leading end of the center electrode and a second leading end thereof, and means for easier adjustment of the spark discharge gap provided at least at any one of portions consisting of an inside edge of the end of the housing, a bridging portion between the end of the housing and the first leading end of the ground electrode, and an outside surface of the ground electrode. The means for easier adjustment of the spark discharge gap is operative in such a manner that the second leading end of the ground electrode comes near the leading end of the center electrode with less force for adjusting the spark discharge gap.
As an aspect of the present invention for realizing a longer life time and a better performance of the spark plug, a noble metal chip or element such as Ir alloy or Pt alloy may be bonded to the second leading end of the ground electrode.
As one of the means for easier adjustment of the spark discharge gap, a relief such as a chamfering or a step is provided at least at an inside edge of the leading end of the housing to which the first leading end of the ground electrode is fixed in order to secure a longer distance to a fulcrum for bending the ground electrode. Therefore, as a length C of the chamfering or the step in the longitudinal direction of the ground electrode is longer, the spark discharge gap adjustment becomes easier.
However, the length C of the chamfering or the step is preferably not longer than 2.0 mm because the length longer than 2.0 mm causes to limit an area where the ground electrode and the housing are bonded and fixed to each other to an extent that bonding strength reliability may not be assured.
As another one of the means for easier adjustment of the spark discharge gap, an intermediate member may be arranged between the leading end of the housing and the first leading end of the ground electrode. The intermediate member is formed in a shape that a cross sectional area S2 of the intermediate member in parallel with the longitudinal direction of the ground electrode is narrower than a cross sectional area (thickness length multiplied by width length) of the ground electrode or is made of material having a lower tensile strength than that of the ground electrode. As a result, the intermediate member may be more easily deformed perpendicular to a longitudinal direction of the ground electrode or to change an angle with the ground electrode.
Further, it is preferable that the cross sectional area S2 is not smaller than 3.0 mm2 and not larger than 8.0 mm2. When the cross sectional area S2 is smaller than 3.0 mm2, heat transfer through the intermediate member gets worse so that temperature of the ground electrode may be unusually increased. On the other hand, When the cross sectional area S2 is larger than 8.0 mm2, an area where the intermediate member is bonded to the housing is too large to secure an adequate bonding strength.
Furthermore, a thickness L2 of the intermediate member is preferably not shorter than 0.5 mm and not longer than 3.0 mm. When the thickness L2 is shorter than 0.5 mm, the intermediate member itself is too short to be deformed by an aimed amount. When the thickness L2 is larger than 3.0 mm, the ground electrode is protruded too deeply into the combustion chamber so that the heat resistance characteristic may be rapidly deteriorated.
In addition, it is preferable that the cross section of the intermediate member is shaped as a rectangular with a short side in parallel with the longitudinal direction of the ground electrode and a long side perpendicular to the longitudinal direction of the ground electrode to have the ground electrode more easily bend. For easily and reliably bonding the intermediate member to the housing, it is also preferable that a length a2 of the short side is not larger than 2.0 mm and a length b2 of the long side is not larger than 4.0 mm.
Material of the intermediate member is preferably a Ni or Fe base alloy having a tensile strength of 350 to 1300 N/mm2.
As further one of the means for easier adjustment of the spark discharge gap, an outside surface or surfaces of the ground electrode are provided with a groove or grooves passing perpendicularly through the ground electrode. Though the groove may be provided in parallel with an adjustment direction of the spark discharge gap so as to narrow the width of the ground electrode at the grooved portion, it is more preferable that the groove is provided perpendicular to the adjustment direction of the spark discharge gap so as to make the thickness of the ground electrode thinner at the grooved portion.
A cross sectional area S3 of the ground electrode at the grooved portion is preferably not smaller than 2.0 mm2. When the cross sectional area S3 is smaller than 2.0 mm2, heat transfer through the ground electrode at the grooved portion gets worse so that temperature of the ground electrode may be unusually increased.
On the other hand, it is preferable that a length P of the groove is not shorter than 0.5 mm and not longer than 2.0 mm. When the length P is shorter than 0.5 mm, an adequate bending of the ground electrode becomes difficult. When the length P is longer than 2.0 mm, an unfavorable restriction of heat transfer through the grooved portion becomes too large. Further, it is preferable that the groove is away by more than 3.0 mm from the second leading end of the ground electrode. Furthermore, material of the ground electrode is preferably a Ni base alloy having a tensile strength of 350 to 1400 N/mm2.